Sheets of aluminum, silver and other metals have long been utilized for insulating articles to intercept radiant heat. Metals are generally susceptible to oxidation and other similar chemical changes due to exposure to the air and therefore, are gradually deprived of surface gloss with lapse of time. Aluminum, for example, undergoes gradual oxidation in the air causing aluminum oxide to form on the surface thereof, with the result that the surface is gradually deprived of gloss over a period of time. When a sheet of a metal is used for insulating an article against heat, the insulating value thereof decreases proportionately as the surface gloss of the metal gradually decreases with passage of time.
In recent years, the so-called metallized plastic films which are obtained by causing a layer of metal such as aluminum, zinc and silver to adhere, by the technique of vacuum deposition or plating, to one surface of a transparent plastic film, such as transparent polyvinyl chloride or transparent polyethylene terephthalate have been used as heat-insulation materials. The composite films which are obtained by adhering a heat-insulation material or reinforcing material such as foam plastic, felt, fabric or metal to the metal surface of metallized plastic films are also used as heat-insulation materials. The metallized plastic films or the composite films are used with their plastic film surfaces facing in the direction of the radiant heat source so as to preclude the previously mentioned problem associated with the use of metal sheets as heat-insulation materials. Since these exterior surfaces are formed of plastic materials such as polyvinyl chloride or polyethylene terephthalate which do not have a satisfactory insulating effect, they tend to become contaminated and and to deteriorate with prolonged use. Therefore, they have the disadvantage that they have a short life especially when used in outdoor services.